1. Field of the Invention
The present invention relates to a method of making magnetic resonance catheter coils employing printed electrical circuit board technology and the flexible catheter coils made therefrom and, more specifically, it relates to miniaturized coils which are sufficiently small as to be insertable into body passageways such as blood vessels, body cavities and the like.
2. Description of the Prior Art
The advantageous use of magnetic resonance technology in providing safe, rapid images of a patient has long been known. It has also been known to employ magnetic resonance technology in producing chemical shift spectra to provide information regarding the chemical content of a material.
In a general sense, magnetic resonance imaging involves providing bursts of radio frequency energy on a specimen positioned within a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei. The emitted signal may be detected in such a manner as to provide information as to the intensity of the response and the spatial origin of the nuclei emitting the responsive magnetic resonance signal. In general, imaging may be performed in a slice or plane or multiple planes or three-dimensional volume with information corresponding to the responsively emitted magnetic radiation being received by a computer which stores the information in the form of numbers corresponding to the intensity of the signal. The image pixel value may be established in the computer by employing Fourier Transformation which converts the signal amplitude as a function of time to signal amplitude as a function of frequency and position. The signals may be stored in the computer and may be delivered with or without enhancement to a video screen display, such as a cathode-ray tube, for example, wherein the image created by the computer output will be presented through black and white presentations varying in intensity or color presentations varying in hue and intensity. See, generally, U.S. Pat. No. 4,766,381.
U.S. Pat. No. 5,170,789 discloses an MR coil probe that is said to be insertable within a specimen, which has an opening, for purposes of nuclear magnetic resonance spectroscopy. It also discloses the use of a probe in the nature of an endoscope. The two component probe has a portion which is insertable into the body cavity and an external portion. As the tuning and matching circuit is outside the body, this limits the permitted extent of insertion into the body. Also, the coil has an elliptical or circular shape that may deform during insertion and, as a result, require that the coil be tuned after insertion. If the coil were made of a very rigid material, insertion problems would also occur. A further limitation of this disclosure is that the coil axis cannot be placed parallel to the direction of the main magnetic field, (denoted herein as the z-axis) otherwise, it would have a practically zero sensitivity. Finally, the coil has no receive-only mode and, as a result, limits its application to spectroscopy. See, also, U.S. Pat. Nos. 4,932,411 and 4,672,972 which have the same inadequacies as the system in U.S. Pat. No. 5,170,789.
U.S. Pat. No. 4,932,411 discloses a solenoidal RF coil which is insertable into the body. The coil, while not disclosed in great detail, is generally similar to the coil of U.S. Pat. No. 5,170,789 except that a solenoidal coil is used instead of a single turn coil.
U.S. Pat. No. 4,672,972 discloses an NMR probe disposed at the distal end of a catheter or endoscope for obtaining NMR spectra from within a patient. The multi-turn probe has a parametric amplifier and/or a gate-array attached to it and also has a coil cooling system. The small parametric preamplifier and the gate-array could tend to create a significant amount of electrical noise to the received signal and, thereby, reduce its sensitivity.
U.S. Pat. No. 5,271,400 discloses the use of an MR active specimen placed in an RF coil within a catheter. The frequency of the signal received by the coil provides information as to the position of the coil. It is not employed to provide MR imaging and spectroscopic analysis. U.S. Pat. No. 5,307,808 has a similar disclosure which employs the signal coming from the surrounding tissue.
One of the beneficial uses of the present invention is in connection with atherosclerotic disease which is a major cause of mortality and morbidity in the United States. Localized forms of the disease, such as the deposit of plaque in the walls of blood vessels, can restrict local blood flow and require surgical intervention in some instances. While x-ray angiography is an established means for detecting the luminal narrowing caused by plaque, it does not provide information regarding the structure of the stenoses nor nature of the process leading to blood flow reduction. Unfortunately, therapeutic methods, such as intravascular intervention, may experience failure partially due to the lack of valid animal models and lack of sufficiently precise imaging methods. An imaging system capable of providing detailed, qualitative and quantitative data regarding the status of vascular walls at the time of surgical intervention, could favorably influence the outcome by enabling the selection of the intervention method to be customized to the particular need. It would also serve to provide precise guidance for various forms of localized therapy.
It has been known to use angioplasty and intravascular ultrasound for imaging plaques. See, generally, Spears et al., "In Vivo Coronary Angioscopy," Journal of the American College of Cardiology, Vol. 1, pp. 395-399 (May, 1993), and Waller et al., "Intravascular Ultrasound: A Histological Study of Vessel During Life," Circulation, Vol., 85, pp. 2305-2310 (1992). Intravascular ultrasound, however, provides several drawbacks, including the insensitivity to soft tissue and the inability to reliably detect thrombus and discriminate thrombus (new or organized) superimposed upon plaque from soft lipid-laden plaques. Also, the presence of artifacts related to transducer angle relative to the vessel wall, calcification of stenoses, and an imaging plane limited to the aperture of the transducer in variable resolution at different depths of view are further problems with this approach.
The feasibility of identification of atherosclerotic lesions by employing MR microimaging in vitro has previously been suggested. See, for example, Pearlman et al., "Nuclear Magnetic Resonance Microscopy of Atheroma in Human Coronary Arteries," Angiology, Vol. 42, pp. 726-733 (1991); Asdente et al., "Evaluation of Atherosclerotic Lesions Using NMR Microimaging," Atherosclerosis, Vol. 80, pp. 243-253 (1990); and Merickel et al., "Identification and 3-d Quantification of Atherosclerosis Using Magnetic Resonance Imaging," Comput. Biol. Med., Vol. 18, pp. 89-102 (1988).
It has also been suggested that MRI can be used for quantification of atherosclerosis. See, generally, Merickel et al., "Noninvasive Quantitative Evaluation of Atherosclerosis Using MRI and Image Analysis," Arteriosclerosis and Thrombosis, Vol. 13, pp. 1180-1186 (1993).
Yuan et al, "Techniques for High-Resolution MR Imaging of Atherosclerotic Plaques,"J. Magnetic Resonance Imaging, Vol. 4, pp. 43-49 (1994) discloses a fast spin echo MR imaging technique to image atherosclerotic plaques on an isolated vessel that has been removed by carotid endarterectomy. As the signal-to-noise ratio (SNR) decreases with the decrease in imaging time and increase in spatial resolution, special RF receiver coils were designed. The article suggests that by the use of special MR hardware at 1.5 T using various T1 and T2-weighted pulse sequences, it is possible to discriminate foam cells, fibrous plaque organized thrombus, new thrombus, loose necrosis and calcium.
It has also been suggested that the fat content of atherosclerotic plaque in excised tissue samples can be determined using chemical shift imaging or chemical shift spectroscopy. See, generally, Vinitski et al., "Magnetic Resonance Chemical Shift Imaging and Spectroscopy of Atherosclerotic Plaque," Investigative Radiology, Vol. 26, pp. 703-714 (1991), Maynor et al., "Chemical Shift Imaging of Atherosclerosis at 7.0 Tesla," Investigative Radiology, Vol. 24, pp. 52-60 (1989), and Mohiaddin et al., "Chemical Shift Magnetic Resonance Imaging of Human Atheroma," Br. Heart J., Vol. 62, pp. 81-89 (1989).
The foregoing prior art articles in the aggregate could lead one skilled in the art to conclude that MR, while having potential for fully characterizing vessel wall disease, suffers from low anatomic resolution unless used in vitro on small specimens with high resolution methods.
MR compatibility characteristics of various catheter and guide wire systems for use in interventional MR procedures, has been considered. See Dumoulin et al., "Real-time Position Monitoring of Invasive Devices Using Magnetic Resonance," Magnetic Resonance in Medicine, Vol. 29, pp. 411-415 (Mar. 1993) and Koechli et al., "Catheters and Guide Wires for Use in an Echo-Planar MR Fluoroscopy System," R. 79th Scientific Meeting, editor, Radiology, Vol. 189 (P), p. 319 (Nov. 1993). It is known that in order to obtain the desired high-resolution imaging and spectroscopy of arteriosclerotic plaques, a coil must be placed close to the target blood vessel.
In Kantor et al., "In vivo .sup.31 P Nuclear Magnetic Resonance Measurements in Canine Heart Using a Catheter-Coil," Circulation Research, Vol. 55, pp. 261-266 (Aug. 1984), there is disclosed an effort to improve the signal-to-noise ratio in the .sup.31 P spectroscopy of a dog myocardium using an elliptical coil. This coil is rigid and rather bulky. Further, as it was designed for spectroscopy of the myocardium, it is not ideal for blood vessels.
Disclosures of efforts to develop catheter coils for imaging vessel walls are contained in Martin et al., "MR Imaging of Blood Vessel with an Intravascular Coil," J. Magn. Reson. Imaging, Vol. 2, pp. 421-429 (1992) and Hurst et al., "Intravascular (Catheter) NMR Receiver Probe: Preliminary Design Analysis and Application to Canine Iliofemoral Imaging," Magn. Reson. Med., Vol. 24, pp. 343-357 (Apr. 1992). These disclosures employ two tiny diameter, back-to-back solenoid coils to produce a good axial profile when the coils are placed along the main magnetic field. The magnetic fields detected by these coils are perpendicular to the long axis of the catheter.
Martin et al., "Intravascular MR Imaging in a Porcine Animal Model," Magn. Reson. Med., Vol. 32, pp. 224-229 (Aug. 1994) discloses use of the system disclosed in the above-cited Martin et al. article for high-resolution images of live animals. See, also, Abstract, McDonald et al., "Performance Comparison of Several Coil Geometries for Use in Catheters," R. 79th Scientific Meeting, editor, Radiology, Vol. 189(P) p. 319 (Nov. 1993). A strong disadvantage of these disclosures is that multislice acquisition cannot be carried out because the longitudinal coverage of the sensitive regions is limited to a few millimeters. Also, these designs require, in order to function effectively, that the long axis of the coils be parallel to the main magnetic field. Unfortunately, for most vessels of interest, such as coronary arteries or veins, for example, the vessels are tortuous and oblique to the magnetic field. Further, to the extent that the coil itself does not have desired flexibility while maintaining the desired efficiency of data acquisition, they are also unsuitable for the purposes of the present invention.
U.S. Pat. No. 5,699,801, assigned to the assignee of the present application, discloses a number of embodiments of flexible coils insertable within small blood vessels of a patient and useful in magnetic resonance imaging and spectroscopic analysis. The coil may be incorporated into an invasive probe and may be introduced into or positioned adjacent to the specimen to be evaluated. The coil may function as a receiver coil having a pair of elongated electrical conductors disposed within an dielectric material and having a pair of ends electrically connected to each other. Associated processing means are disclosed. The disclosure of this patent is expressly incorporated herein by reference.
There remains, therefore, a very real and substantial need for an improved means for MR imaging and spectroscopic analysis of specimens in a manner which provides efficient data acquisition with maximum SNR while permitting in vivo or in vitro acquisition from small vessels, as well as other body openings and a wide range of other types of specimens.